Choroid plexus histidine transport.
System-N transport plays an important role in l-glutamine uptake into isolated rat choroid plexus but its role in the transport of another System-N substrate, l-histidine, has yet to be determined. Similarly, the possible effects on System-N mediated l-histidine transport of changes in pH and extracellular l-glutamine, such as occur in cerebral ischemia and hepatic encephalopathy, have yet to be examined. In the absence of competing amino acids, l-[3H]histidine uptake in isolated rat choroid plexus was mediated by both Na+-independent and Na+-dependent transport. The former was inhibited by 2-amino-2-norbornane carboxlic acid, indicating System-L transport, while the latter appears System-N mediated as it was inhibited by three System-N substrates but not substrates for System-A and -ASC. The Na+-dependent uptake had a Km of 0.2 mM and a Vmax of 1.4 nmol/mg/min. It accounted for 30% of l-histidine uptake in the presence of physiological concentrations of amino acids. Reductions in pH markedly inhibited Na+-dependent but not Na+-independent transport indicating that, as in liver but not neurons, System-N mediated transport at the choroid plexus is pH sensitive. Increases in l-glutamine concentration in the pathophysiological range reduced l-histidine uptake via both System-L and -N.
Xiang
J
J
Department of Surgery (Neurosurgery), University of Michigan, R5605 Kresge I, Ann Arbor, Michigan, MI 48109-0532, USA.
Fowkes
R L
RL
Keep
R F
RF
D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002831:Choroid Plexus / Q000378:metabolism*; D005973:Glutamine / Q000378:metabolism; D006639:Histidine / Q000493:pharmacokinetics*; D006863:Hydrogen-Ion Concentration; D016014:Linear Models; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley
D005973:Glutamine; D006639:Histidine